Radiation detectors

ABSTRACT

A radiation detector has a dual-capacitive structure comprising an array A of first capacitors (10) including discrete electrodes (13) arranged in rows and columns and a second capacitor (20). The second capacitor (20) incorporates a radiation converter (21) which co-operates with the first capacitors (10) to cause an accumulation of charge on the discrete electrodes (13) according to the spatial distribution of radiation to which the radiation converter is exposed. Read-out means (30) is provided to output a signal representative of the accumulated charge.

BACKGROUND OF THE INVENTION

This invention relates to radiation detectors and in particular tolarge-area, two-dimensional, pixellated radiation detectors.

The invention has particular, though not exclusive application to suchdetectors used for medical imaging; for example, medical X-radiationimaging.

Pixellated radiation detectors usually comprise an array ofradiation-sensitive elements layed out in two-dimensional fashion.Typically, the elements are arranged in rows and columns to form aregular array.

In one known example of such radiation detectors each element in thearray comprises a reverse-biased photodiode coupled to a transistor.During signal integration, the transistor is held in a non-conductingstate and an optical radiation signal derived from the incidentradiation is allowed to discharge the bias applied across thephotodiode. The signal is then readout by switching the transistor toits conducting state and recording the amount of charge required torestore the reverse bias across the photodiode. Such arrays aretypically constructed from crystalline silicon or hydrogenated amorphoussilicon (a - Si:H).

A radiation detector of this kind tends to be unsatisfactory in that thedetector exhibits non-linear performance and saturation for large inputsignals, and leakage current in an a-Si:H sensor results in appreciabledetected shot noise. Also, such radiation detectors suffer from imagelag due to trapping sites in the semiconductor material.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a radiationdetector comprising, first capacitive means including a two-dimensionalarray of discrete, charge-collection electrodes, second capacitive meansincluding radiation conversion means, the second capacitive means beingco-operable with the first capacitive means to cause charge toaccumulate on said discrete, charge-collection electrodes according tothe spatial distribution of intensity of radiation to which saidradiation conversion means is exposed, and read-out means for outputtinga signal representative of charge that accumulates on the discrete,charge collection electrodes of the first capacitive means.

The radiation detector has a dual-capacitive structure which serves toprotect the read-out circuitry from exposure to relatively highelectrical voltages associated with the operation of the radiationconversion means. Also, the structure of the radiation detector enablesa relatively high frame rate imaging capability to be attained.

According to another aspect of the invention there is provided aradiation detector comprising, first capacitive means including asemiconductor substrate and a two-dimensional array of discrete,charge-collection electrodes formed on a first surface of thesemiconductor substrate, each said charge-collection electrode formingpart of a respective first capacitor,

second capacitive means comprising a second capacitor arranged in serieswith the first capacitors of the first capacitive means, the secondcapacitor comprising a layer of a radiation conversion material forconverting radiation to which the layer is exposed into electricalcharge and focussing means for focussing charge generated in the layerof radiation conversion material onto said discrete charge-collectionelectrodes, whereby charge accumulates on the charge-collectionelectrodes substantially according to the spatial distribution of theintensity of said radiation,

and read-out means for outputting a signal representative of charge thataccumulates on the discrete, charge-collection electrodes.

DESCRIPTION OF THE DRAWINGS

Radiation detectors according to the invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

FIG. 1 illustrates diagrammatically a radiation detector according toone embodiment of the invention;

FIGS. 2(a) to 2(c) show the equivalent circuits of differentimplementations of the radiation converter shown in FIG. 1;

FIG. 3 shows diagramatically the read-out circuitry of the radiationdetector illustrated in FIG. 1;

FIGS. 4a to 4d illustrate how the radiation detector is sequenced;

FIG. 5 is a simplified plan view of a group of four charge-collectionelectrodes in a radiation detector according to another embodiment ofthe invention;

FIG. 6 shows a simplified cross-sectional view through part of theradiation detector shown in FIG. 5;

FIG. 7 shows an idealised distribution of the potential minimum forelectronic focussing in the radiation detector shown in FIGS. 5 and 6;

FIG. 8 is a simplified cross-sectional view through part of a radiationdetector showing the radiation converter; and

FIG. 9 is a diagrammatic illustration of an imaging system incorporatinga radiation detector according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, the radiation detector includes adual-capacitive structure comprising an array A of first capacitors 10and a second capacitor 20, and further includes semiconductor read-outcircuitry 30.

The second capacitor 20 incorporates a radiation converter 21. The formof radiation converter used will depend upon the particular applicationto which the radiation detector is being put. In this particularembodiment, the radiation detector is suitable for the detection ofX-radiation with high resolution, and the radiation converter 21comprises a flat plate electrode 22 made from a suitableradiation-transmissive material and an ionisation medium 23, which couldbe a solid, liquid or a gas, contained within an ionisation chamber (notshown).

The array A of first capacitors 10 is formed by a thin layer 11 of adielectric material disposed between a planar electrode 12 and atwo-dimensional array of discrete, charge-collection electrodes 13arranged in rows and columns (of which only a single row is shown inFIG. 1). Each discrete electrode 13 forms part of a respective saidfirst capacitor 10 in the array A.

In a typical embodiment, layer 11 might comprise a sheet of glass fibrecomposite forming part of a printed circuit board with metal padsforming the electrode array 13 and a ground plane forming the planarelectrode 12.

As will be described in greater detail hereinafter, electrode 22 of theradiation converter presents a surface which is exposed to radiationbeing detected, and each discrete electrode 13 corresponds to a singlepixel in the image of the detected radiation which is to be formed bythe radiation detector.

Referring now to the equivalent circuits shown in FIGS. 2a-2c, electrode22 is connected to an HT voltage source 40. The arrangement is such thateach capacitor 10 is connected in series to capacitor 20. For clarity,FIGS. 2a-2c show only one of the capacitors 10 in the array A.

The HT voltage source 40 supplies a voltage V_(EHT) to electrode 22 andcreates a strong electric field E_(o) across the ionisation medium 23.

In operation of the radiation detector, incident radiation ionises theionisation medium 23 creating electrons and positive ions, or in thecase of a semiconductor, electrons and holes. These charges will driftin the direction of the applied electric field, E_(o), to electrodes 13and 22. When +V_(EHT) is connected to electrode 22, electrons will driftto electrode 22 whereas the positive ions or holes are caused to driftto the discrete electrodes 13.

The charge which accumulates on the array of discrete electrodes 13corresponds substantially to the spatial distribution of the intensityof incident radiation.

It is the function of the read-out circuitry 30 to output a signalrepresentative of charge that has accumulated on electrodes 13, whichsignal can then be processed to generate a two-dimensional imagerepresenting the spatial distribution of intensity of the incidentradiation. To that end, each electrode 13 is connected to a respectivedata line (D) via a respective semiconductor switch in the form of afield effect transistor (FET or JFET) 31 and an optional amplifier 32.

FIG. 2(a) shows the simplest implementation in which the amplifier 32 isomitted, whereas FIGS. 2(b) and 2(c) show more complex implementationsin which the amplifier is included. Each amplifier 32 has an associatedreset switch 33 and output multiplexer switch 34. FIG. 2(b) shows thearrangement for 8 voltage sensitive amplifier whereas FIG. 2(c) showsthe arrangement for a current sensitive amplifier.

With the respective switch 31 open, each first capacitor 10 in array Aintegrates part of the total signal current i_(sig) (t). Since

    i.sub.sig (t)=i.sub.1 (t)+i.sub.2 (t),                     (1)

where

i₁ (t) is the current flowing through the respective capacitor 10 attime t, and

i₂ (t) is the current flowing through capacitor 20, then respectivevoltages V₁, V₂ are developed across each capacitor 10 and acrosscapacitor 20 according to the expression ##EQU1## where t_(i) is thetime interval during which charge accumulates on the discrete electrodes13 i.e. the integration period,

C₁ is the capacitance of the first capacitors 10 and

C₂ is the capacitance of the second capacitor 20.

The respective voltage V₁ developed across each first capacitor 10during the integration period is t_(i) is proportional to the integratedintensity of radiation received at the corresponding pixel during thatperiod. Accordingly, by reading out the voltages V₁, an image of theintensity distribution of incident radiation can be derived.

In order to maximise the detected output signal, the current i₂ flowingthrough the second capacitor 20 should be as small as is practicable. Itfollows from equation (2) above that,

    i.sub.1 /i.sub.2 =-C.sub.1 /C.sub.2                        (3)

and so the value of C₁ should be much larger than the value of C₂.Typically, the ratio of C₂ :C₁ should be in the range 5 to 10,000, andpreferably in the range 100 to 10,000. This can be achieved by makinglayer 11 as thin as possible, and by forming the layer from a materialwhich has a high dielectric constant.

Also, the time constant τ of the circuit formed by FET 31 in its offcondition and by the first capacitors 10 should be large relative to thesum of the integration period t_(i) and the read-out period t_(r)whereby to reduce the possibility of signal loss caused by the firstcapacitors 10 discharging through FET 31.

The time constant τ is given by the expression

    τ=R.sub.o C.sub.1                                      (4)

where R_(o) is the resistance of FET 31 when it is off.

Accordingly, the optimum conditions prevail when the values of R_(o) andC₁ are both large. It is also advantageous to reduce the read-out periodt_(r) of the detector.

As will now be explained, the switching condition of each FET 31 is socontrolled as to determine the order in which the voltages V₁ are readout from the first capacitors 10 in array A.

As shown in FIGS. 2a to 2c, the source of each FET 31 is connected tothe associated discrete electrode 13, whereas the drain is connected toa respective data line D via the optional amplifier circuit 32.

FIG. 3 illustrates diagramatically how the read-out circuitry isarranged when, as described with reference to FIG. 2(a), the amplifiercircuits 32 are omitted.

In this case, FETs 31 connected to electrodes 13 in the same row (ROW 1,say) have their gates connected to the same control line (C 1), andsimilarly, FETs connected to electrodes 13 in the same column (COL 1,say) have their drains connected to a common data line (D 1) which, inturn, is connected to a common processing circuit (PRO 1). With thisarrangement, voltages V₁ developed across the capacitors 10 in aselected row (ROW 1, say) can be read out by setting the control line(C 1) for that row HIGH and setting the other control lines (C 2, C 3 .. .) LOW. In this condition, the FETs in the selected row will all beclosed, whereas all the other FETs, associated with the other,non-selected rows will be open. Therefore, the signals on the respectivedata lines D for the selected row will represent the voltages V₁developed across the respective capacitors 10 in that row and can beread out by external processing circuits PRO 1, PRO 2 . . . eithersequentially, column-by-column, or simultaneously, in multiplex fashion.

This procedure is then repeated for the other rows in the array untilthe voltages V₁ across all the capacitors 10 have been read out.

The external processing circuits PRO 1, PRO 2 . . . may be eithercurrent sensitive or voltage sensitive.

When amplifier circuits 32 are included, the read-out circuitry will bearranged differently. In this case, a global gate control signal GATE isapplied simultaneously to all the FETs 31 in the array, whereas thereset switch 33 and the output multiplexer switch 34 associated witheach amplifier 32 in the same row are controlled via a commonmultiplexer control line C. Output signals on the respective data linesD for a selected row are then read out column-by-column or in multiplexfashion, as before.

FIGS. 4(a) to 4(d) illustrate diagrammatically, by way of example, howthe radiation detector is sequenced in order to accumulate charge inresponse to incident radiation and to read out the charge stored on thecapacitors 10. There are four stages in each sequence, as follows:

Stage 1 (FIG. 4a)

The radiation beam is turned on and the FETs 31 are all opened such thateach discrete electrode 13 is in effect floating from ground potential.

The incident radiation ionises the ionisation medium 22 of the radiationconverter 21 causing, for example, positive ions (+) to accumulate onthe discrete electrodes 13.

Stage 2 (FIG. 4b)

Momentarily before reading the charge stored on the electrodes 13, thereset switch 33 is opened.

Stage 3 (FIG. 4c)

The transistor 31 is then closed, thus allowing charge stored on theelectrode to reach the external processing circuits, PRO1, PRO2 . . .The signal at the output of the processing circuit is proportional tothe radiation intensity during the integration period.

Stage 4 (FIG. 4d)

By closing switch 33 the electrode 13 is connected to ground potential,thus resetting the pixel ready for the next integration period (stage1).

Every pixel on the array is capable of storing charge at all points inthe readout cycle, except in stage 4. Therefore, the detector may beused to accumulate a single image in which the radiation beam is turnedon and off again within stage 1 only, or it may be operated in acontinuous (cine) mode by continuously cycling the detector through thefour stages in sequence with the beam being turned on continuously.

The dielectric layer 11 and at least some of the read-out circuitry 30associated with the array A of first capacitor 10 may advantageously befabricated from a common substrate of semiconductor material, such ashydrogenated amorphous silicon (a-Si:H) or polysilicon (p-Si). Thediscrete charge-collection electrodes 13 which, in effect, define thesize and pitch of the pixels in the detector may be formed on a surfaceof the semiconductor substrate by evaporation, or any other suitabledeposition technique, and, in one implementation, associated circuitcomponents (e.g. FET 31) may be buried in the substrate materialimmediately below the respective electrode 13. The control lines C andGATE and data lines D which interconnect the circuit components extendout to adjacent edges of the substrate for connection to the associatedprocessing circuitry. This configuration in which the circuit componentsare located beneath each pixel-defining electrode 13, has the advantagethat the spacing between electrodes can be reduced significantly, withthe result that the detector will be sensitive to radiation incidentover more than 90% of the exposed detector area even though the pixelsthemselves may be relatively small, typically less than 100 μm across.

It is known that the resistance R of an a-Si:H FET is given by theexpression

    1/R=(W/L) μ.sub.FE (V.sub.G -V.sub.T) C.sub.G           (5)

where W and L are the width and the length respectively of the FET,

μ_(FE) is the carrier mobility,

V_(T) is the threshold voltage,

V_(G) is the gate voltage, where V_(G) V_(T), and

C_(G) is the gate capacitance.

When the FET is `closed` (i.e. the low resistance, `read-out` state),the above parameters might typically have values as follows:

W/L=10

μ_(FE) =0.5 cm⁻² V⁻¹ sec⁻¹

V_(T) =1V

V_(G) =5V

C_(G) =10⁻⁸ Fcm⁻².

Applying these values to equation 5 above, the FET resistance R_(on)during read-out would be approximately 5MΩ.

In the case of a detector having a relatively small pixel size, 100 μmsquare, say and a dielectric layer 11 one micrometer thick, thecapacitance C₁ of each capacitor 10 would be 500 fF. Accordingly, theread-out time constant τ_(R) (=R_(on) ·C₁) for a pixel in the arraywould be about 2.5 μsec so that in the case of an array consisting of2000 rows, the read-out period for the entire array would be about 25msec, corresponding to a peak frame rate of 40 frames sec⁻¹, thisallowing a period of 5 τ_(R) to read-out data from all the pixels in arow. Higher frame rates than this could, of course, be achieved byincreasing the ratio W/L of the FET, and/or by increasing the gatevoltage V_(G).

A problem associated with electrical circuitry fabricated insemiconductor materials, such as a-Si:H is its susceptibility toelectrical breakdown due to exposure to high electrical voltages.

However, the radiation detector described with reference to FIG. 1 has anumber of beneficial features that are designed to protect the read-outcircuitry from damage which it might otherwise suffer both during theintegration stage, while electrical charge is accumulating on thediscrete electrodes 13, and also during the read-out stage.

The semiconductor material in layer 11 has a high dielectric constantand so, because of the ratio of capacitances, protects the read-outcircuitry 30 from electrical discharge that could occur in the radiationconverter 21. In addition, optional back-to-back diodes (D1,D2) may beconnected across each FET 31 to provide additional protection,preventing a build-up of large electrical signals in the event ofdetector flashover. The high resistivity of layer 11 also ensures lowleakage current and so low shot noise at each pixel.

Furthermore, the voltage (V_(EHT) -V₁) developed across the capacitor 20will generally be much larger than the voltage V₁ developed across thecapacitor 10 and this ensures that the voltage across the radiationconverter 21 is maintained substantially constant, resulting in goodsignal linearity over a wide range of integrated input signal.

The radiation detector described with reference to FIG. 1 incorporates aradiation converter 21 in the form of an ionisation medium, and issuitable for the detection of X-radiation with high resolution. Goodspatial resolution is obtained due to the high electric field E_(o)within the ionisation medium caused by the applied voltage V_(EHT). Thisfield tends to confine charged particle motion to movement in adirection perpendicular to the plane of electrodes 13 and 22. Sincepositive ions typically have lower mobility than electrons in ionisationmedia, spatial resolution will usually be maximised when positive ionsare caused to drift to electrode 13.

FIGS. 5, 6 and 7 illustrate a preferred embodiment of the invention inwhich the dual capacitive structure is fabricated from hydrogeneratedamorphous silicon (a-Si:H), although polysilicon (p-Si) couldalternatively be used. As in the case of the embodiments described withreference to FIGS. 1 to 4, the dual capacitive structure comprises atwo-dimensional array of first capacitors 10, each having a discretecharge-collection electrode 101 at which charge can accumulate, and acommon, second capacitor 20 overlying, and connected in series with, thefirst capacitors 10.

FIG. 5 is a simplified plan view showing a group of fourcharge-collection electrodes 101 which form part of a largertwo-dimensional array- Each electrode 101 forms part of a respectivecapacitor 10, and FIG. 6 shows a cross-sectional view through theradiation detector, taken on line A--A in FIG. 5, through only one ofthe capacitors 10.

Referring to FIG. 6, each capacitor 10 comprises a layer 102 of a-Si:H:Nbearing a charge-collection electrode 101 on one surface S₁, thereof,and a corresponding pixel electrode 103 at the opposite surface S₂,directly below electrode 101. The pixel electrode is connected to groundpotential.

Electrode 101 is connected to a data line D via a respective switchingcircuit comprising a field effect transistor 31 formed in layer 102alongside the associated capacitor 10.

Each transistor 31 comprises a layer 131 of a-Si:H formed on surface S₁of layer 102 and regions 132,133 of n⁺ -type material (a-Si:H:n⁺)deposited on layer 131. Each such region 132,133 has a respectiveelectrical contact; a contact 134 (the source electrode) connected toelectrode 101 and a contact 135 (the drain electrode) connected to therespective data line D. The transistor 31 also has a gate electrode 136formed at surface S₂ of layer 102, this being connected to therespective control line C.

The second capacitor 20 of the dual capacitive structure comprises alayer 201 of intrinsic a-Si:H which overlies the discrete electrodes 101and their associated field effect transistors 31, and a layer 202 of n⁺-doped material (a-Si:H:n⁺) which is connected to a source of HT voltageand forms the top plate of the second capacitor 20.

A layer (not shown) of insulating material would also be provided toisolate the transistor and the data line from the layer 201.

In this embodiment, the radiation detector also comprises a layer 300 ofa scintillator, caesium iodide (CsI) having a columnar structure. Layer300 does not form part of the dual capacitive structure 10,20, butconverts radiation that is to be detected (e.g. x-radiation orγ-radiation) to optical radiation, the intensity of optical radiationproduced being dependent on the intensity of radiation to which layer300 is exposed.

Optical radiation produced in this way, passes through layer 300 andinto layer 201 of the second capacitor 20. Any spreading of the opticalradiation in layer 300 is limited by the columnar, crystalline structureof the caesium iodide so that the spatial distribution of opticalphotons passing into layer 201 substantially matches the spatialdistribution of the intensity of radiation to which layer 300 isexposed.

Individual photons entering layer 201 create electron-hole pairs in thesemiconductor material of the layer. In this embodiment, holes willdrift towards layer 202 formed of n⁺ -type material whereas electronswill drift towards a respective charge-collection electrode 101. Ineffect, the second capacitor 20 functions as a semiconductor driftchamber.

There is also provided on surface S₁ a thin, narrow layer 203 ofheavily-doped n⁺ -type material (a-Si:H:n⁺) which substantiallysurrounds each charge-collection 101 and its associated field effecttransistor 31. Also, a layer 204 of heavily-doped p⁺ -type material(a-Si:H:p⁺) is deposited on each electrode 101. The spatial distributionof layers 203,204 is such as to create potential minima P_(m) within thesecond capacitor 20, as shown in idealised form in FIG. 7, so thatcharge (in this embodiment electrons) produced within layer 201 isconstrained to move within a potential well and is thereby focussed ontothe respective charge-collection electrode.

It will, of course, be appreciated that the polarities of layers 202,203and 204 could be reversed in which case holes will drift towards thecharge collection electrodes 101 and electrons will drift towards layer202.

The structure described with reference to FIGS. 5 to 7 has significantattributes.

Each switching circuit (FET 31) is positioned alongside, and insubstantially the same plane as, the associated capacitor 10.Accordingly, layer 102 can be made relatively thin, typically about 300nm, and so each capacitor 10 will have a much larger capacitance thanthat of capacitor 20, giving high dynamic range and linearity. With thisconfiguration, the area of each charge-collection electrode 101 must bereduced in size in order to accommodate the associated FET.Nevertheless, by means of electronic focussing, as described, thedetector can still attain a high charge-collection efficiency (thedetector can be made sensitive to radiation incident on approximately100% of the exposed detector area), and also provides excellentcross-talk rejection between neighbouring pixels.

The charge which accumulates on electrodes 101 in each row can be readout as before by controlling the data and control lines D,C. Asdescribed with reference to FIG. 3, the pixels in each row can be readsequentially, column by column or simultaneously in mulitplex fashion.

The structure described with reference to FIGS. 5 to 7 can be fabricatedusing well known amorphous silicon processing techniques.

Typically, an aluminium film is deposited on a flat substrate, made fromglass for example, and is etched to form the gate electrodes 136, theassociated control lines C and the pixel electrodes 103.

A layer of amorphous silicon nitride (a-Si:H:N), typically 300 nm thick,is then deposited on the etched film, followed by a thin layer ofintrinsic a-Si:H and then a thin layer of n⁺ -type material (a-Si:H:n⁺).The layers of a-Si:H and n⁺ -type material are then etched awayphotolithographically to form layers 131 and the source and drainregions 132,133 of the transistors 31. A second aluminum film isdeposited over the etched semiconductor material and this is then etchedaway to form the charge-collection electrodes 101, the source and drainelectrodes 134,135 and the data lines D. A plasma etch process is thenused to remove extraneous n⁺ -type material in the channel of thetransistor 31.

Further layers of n⁺ -type and p⁺ -type material are deposited andetched photolithographically to form layers 203,204 used for electronicfocussing. The detector is then completed by depositing an approximately2 μm thick layer of intrinsic a-Si:H (layer 201) followed by a thinlayer of n⁺ -type material (layer 202) on which a contact electrode isdeposited by electrodeposition, and finally a 300 μm thick layer ofcolumnar caesium iodide is deposited by chemical deposition and/or silkscreen printing (layer 300).

It will be appreciated that the structure described with reference toFIGS. 5 to 7 is highly adaptable in that the same semiconductorsubstrate incorporating an array of capacitors 10, and their associatedfield effect transistors, control and data lines can be used inconjunction with a wide range of different radiation converters.

In an alternative embodiment, layers 201, 202 and 300 in FIG. 6 could bereplaced by a single layer of a radiation conversion material whichconverts radiation that is to be detected directly into electron-holepairs. An example of an especially useful material for this purpose iscadmium telluride (CdTe) which can be used to detect X-radiation,γ-radiation, α- or β-particles.

Cadmium Telluride is deposited by electrodeposition and if an abundanceof ¹¹³ Cd isotope is provided the resulting layer will be renderedsensitive to neutrons.

In a yet further embodiment, layers 201, 202 and 300 could be replacedby a liquid or gas or solid ionisation chamber similar to that describedwith reference to FIG. 1.

As will be appreciated from the described embodiments, the radiationdetector provides a large-area, high resolution image well suited tomedical imaging (e.g. X-radiation imaging). However, it will beunderstood that the invention is not limited to this application, and bysuitable choice of radiation converter detectors according to theinvention find a wide range of different imaging applications.

In another application of the invention, the radiation detector is usedfor ₁ H³ autoradiography.

In this case, the radiation converter 20 comprises a thin photocathodeevaporated onto the top surface of a microchannel plate. After labellingand electrophoresis, a gel under investigation is impregnated in ascintillation liquid before being placed on the photocathode of theradiation converter. β⁻ particles emitted by the ₁ H³ label cause thescintillation liquid to emit photons in the form of visible light which,in turn, liberate electrons from the photocathode. The photoelectronsenter the microchannel plate and are accelerated towards the array ofcharge-collection electrodes (13,101) where the negative chargeaccumulates. The electrodes (13,101) could be supported on asemiconductor substrate as described with reference to FIGS. 5 to 7. Thegain of the microchannel plate assists in enhancing the signal-to-noiseratio of the detector output signal.

As in the embodiment described with reference to FIGS. 1 to 7, thecapacitance C₂ of the radiation converter is much less than that ofcapacitance C₁, and so the latter is effective to protect the read-outcircuitry 30 from high electrical voltage associated with the operationof the converter.

In another application of the invention, the radiation detector is usedto image neutrons. ₆₄ Gd¹⁵⁸ is a stable nuclide with a natural abundanceof -25% and a cross-section to thermal neutrons of 2.6×10⁵ barns. Itundergoes a (n, γ) reaction to produce ₆₄ Gd¹⁵⁷ (also a stable nuclide),with an energy of -7.9 MeV.

The reaction also produces low energy ≦41 keV Auger electrons. It isthese Auger electrons which are used to form the neutron image using aradiation converter similar to that used for detecting the ₁ H³ labeldescribed hereinbefore. However, in this instance, a thin scintillationlayer would be applied directly to the photocathode instead of using ascintillation liquid. A layer of ₆₄ Gd¹⁵⁸ would overlie and protect thescintillation layer and photocathode.

A neutron detector of this kind could be used in combination with a lowenergy (≦500 keV) X-ray detector. Typically, the X-ray detector wouldhave a radiation converter comprising an ionisation medium adapted forhigh resolution, similar to that described with reference to FIG. 1. Itwould be necessary to ensure that sensitivity to high energy (≦1 MeV)X-rays is low in order to reduce contamination from the Gd neutronconverter. However, since the two converters are so closely linked itwould be possible to generate "neutron contamination correction factors"to compensate for this effect.

In a yet further application of the invention, the radiation detectormay be used for ₁₆ S³⁵ auto-radiography. ₁₆ S³⁵ emits β⁻ particles withan energy of 168 keV capable of penetrating a thin sheet of metal, suchas beryllium.

In this case, the radiation converter 21 may comprise a liquidionisation chamber, filled with suitable ionisation liquid such asiso-octane and having a window made from a thin sheet of beryllium. Bysealing the chamber it would be possible to apply some pressure to theberyllium window for cleaning without deforming the chamber too greatly.Preferably, however, a thin film of TEFLON polytetrafluoroethylene film(E.I. du Pont de Nemours, Wilmington, Del.) or other tough plasticssheet would be stretched over the window to reduce operator damage, withlittle effect on detector sensitivity.

In order to increase the gain of the radiation converter, a wiremulti-step avalanche stage or other high gain facility could beinstalled in the ionisation space of the ionisation chamber.

With a view to facilitating cleaning of the radiation converter, a solidionisation medium such as a thick layer of a-Si:H or other amorphoussemiconductor material could be used. Alternatively, an opticallysensitive radiation converter as described hereinbefore for ₁ H³auto-radiography could be used.

As the above-described embodiments show, a radiation detector inaccordance with the invention can be used to detect a range of differentkinds of radiation including electromagnetic radiation (e.g.X-radiation, γ-radiation, optical radiation), charged particles (e.g.α-particles, β- particles), neutrons and also pressure waves andmagnetic field.

In a further application of the invention, for imaging low energy α, β,X- and γ-radiation, the converter 23 could comprise a semiconductingpolymer film using materials such as polypyrrole or polyaniline. In thiscase, the radiation could interact with the polymer which would allowsignal transport to the charge collection electrodes. High atomic numberelements could be added to the polymer substrate to improve theinteraction probability of high energy X- and γ-rays. A furtheralternative would be to substitute polymer materials whose conductivityvaries as a function of chemical concentration, thus forming apixellated "electronic nose".

Accordingly, the radiation converter may take a variety of differentforms depending upon the application to which the radiation detector isbeing put, including converters incorporating:

(a) group IV semiconductors (e.g. silicon and germanium in crystalline,polycrystalline or amorphous form). These materials are useful fordetecting α, β, γ, X-ray or optical radiation.

(b) compound semiconductors including GaAs, CdTe, HgI₂, TlBr incrystalline, polycrystalline or amorphous form. These materials areuseful for detecting α, β, γ, X-ray or optical radiation.

(c) semiconducting polymers.

(d) amorphous selenium. This material is useful for detecting α, β, γand X-ray.

(e) gas ion chambers (e.g. Ar, Xe, Kr gases either pure or mixed withsuitable quench agents at room or high pressure). Such converters couldbe used to detect α, β, γ or X-radiation.

(f) gas proportional chambers (e.g. Ar, Xe, Kr gases with suitablequench agents). Such converters could be used to detect α, β, γ orX-ray.

(g) liquid ion chambers (e.g. pure alcohols). Such converters could beused to detect α, β, γ or X-ray.

(h) scintillators combined with photocathodes having an electronaccelerating gap forming the dielectric of capacitor 20. A converter ofthis kind could be used to detect α, β, γ or X-radiation.

(i) scintillators irradiating semiconductor layers, where thesemiconductor layer forms the capacitance C₂.

Converters such as this could be used to detect α, β, γ or X-radiation.

(j) Metal foils with high neutron interaction cross-section in contactwith a device of type (a), (b), (c), (d), (e), (f), (g), (h) or (i).Such converters could be used to detect neutrons.

(k) Piezo-electric material. This material is sensitive to pressurewaves. The interacting wave causes a voltage change across thepiezo-electric material which forms the conversion medium 20. Thischange in voltage causes a current to flow in C₁ thereby generating theimaging signal.

(l) Semiconductor material operated as a Hall effect device. Suchdevices are sensitive to magnetic field. The generation of a Hall effectvoltage across the semiconductor used as the conversion medium 20 causesa current to flow in C₁ thereby generating the imaging signal. Thesealternative forms of radiation converter are representeddiagrammatically by the component referenced RC in FIG. 8.

In almost every case, the radiation converter generates chargedparticles in response to incident radiation causing charge to accumulateon the charge-collection electrodes of capacitors 10, the resultingvoltages V₁ developed across capacitors 10 being read-out using read-outcircuitry.

A particular advantage of the described radiation detectors is thatwhile the radiation converter may have a variety of different forms, asdescribed herein, the structure defining the capacitors 10 and thereadout circuitry 30 with which the converter is used requires nosignificant modification. Accordingly, experimentation using differentkinds of radiation converter is relatively straightforward.

The radiation detectors which have been described have the same generalstructure, namely an array of discrete electrodes each defining arespective pixel in the image that is to be formed and comprising partof a respective first capacitor, a second capacitor incorporating aradiation converter and read-out circuitry for reading out a signalrepresentative of charge that accumulates on the discrete electrodes ofthe first capacitors in response to radiation detected by the radiationconverter.

As already explained, a dual capacitive structure such as this isadvantageous in that the capacitance of the first capacitor protects thereadout circuitry from exposure to relatively high electrical voltagesused in the operation of the radiation converter. Also, when applied tomedical imaging, the radiation detector provides the combination of highspatial resolution (due to the quasi one-dimensional path of chargecarriers generated in the radiation converter), and high quantumefficiency (due to the relatively large detector thickness possiblecompared to the scintillation phosphors used with X-ray film and otherpixellated detectors).

Also, with an appropriate choice of radiation converter, the describedradiation detectors provide a high frame rate imaging capability withlittle or no image lag. This cannot currently be achieved using knownamorphous silicon (a-Si:H) detectors due to the number of trappingstates in the a-Si:H itself. A radiation detector according to theinvention may be incorporated in an imaging system, such as a medicalX-radiation imaging system, as shown diagrammatically in FIG. 9.

INDUSTRIAL APPLICABILITY

The invention is applicable to large-area, two-dimensional, pixellatedradiation detectors particularly, though not exclusively, for medicalimaging e.g. medical X-radiation imaging.

I claim:
 1. A radiation detector comprising,a plurality of firstcapacitors forming a two dimensional array, each said first capacitorhaving a discrete, charge-collection electrode, a second capacitorhaving a radiation-receiving surface, the second capacitor includingradiation conversion means for converting radiation received at saidradiation-receiving surface to electrical charge, each said firstcapacitor being connected electrically in series with said secondcapacitor and having a capacitance greater than the capacitance of saidsecond capacitor, charge-focussing means for focussing said electricalcharge produced by said radiation conversion means onto the discrete,charge-collection electrodes of said plurality of first capacitors,whereby, in operation, electrical charge accumulates on saidcharge-collection electrodes substantially according to the spatialdistribution of the intensity of radiation received at saidradiation-receiving surface, and read-out means for outputting an outputsignal representative of the accumulated charge on the discrete,charge-collection electrodes of said plurality of first capacitors.
 2. Aradiation detector as claimed in claim 1 wherein the ratio of thecapacitance of each said first capacitor to the capacitance of saidsecond capacitor is in the range from 10,000 to
 5. 3. A radiationdetector as claimed in claim 2 wherein the ratio of the capacitance ofeach said first capacitor to the capacitance of said second capacitor isin the range from 10,000 to
 100. 4. A radiation detector as claimed inclaim 1, wherein said plurality of first capacitors includes a layer ofdielectric material, and said discrete, charge-collection electrodes areformed on a surface of said layer of dielectric material.
 5. A radiationdetector as claimed in claim 1, wherein said plurality of firstcapacitors comprises a layer of semiconductor material and saiddiscrete, charge-collection electrodes are formed on a first surface ofsaid layer of semiconductor material.
 6. A radiation detector as claimedin claim 5 wherein said semiconductor material is hydrogenated amorphoussilicon (a-Si:H) or polysilicon (p-Si).
 7. A radiation detector asclaimed in claim 5 wherein said read-out means comprises a plurality offield effect transistors each for connecting a respective saidcharge-collection electrode of said plurality of first capacitors to adata line, each said field effect transistor comprising a drain and asource formed on said first surface of said layer of semiconductormaterial adjacent to the respective said charge-collection electrode towhich the field effect transistor is electrically connected.
 8. Aradiation detector as claimed in claim 7 wherein each said field effecttransistor has a gate electrode formed on a second surface of said layerof semiconductor material.
 9. A radiation detector as claimed in claim 8wherein the gate electrodes of said field effect transistors areconnected to control lines for enabling said read out means to outputsaid output signal row-by-row or column-by-column.
 10. A radiationdetector as claimed in claim 9 wherein said read-out means comprisesmeans for outputting said output signal from different rows (or columns)in said array either sequentially, column-by-column (or row-by-row) orin multiplex fashion.
 11. A radiation detector as claimed in claim 7wherein said charge-focussing means includes a body of semiconductormaterial formed on said first surface of said layer, said body ofsemiconductor material substantially surrounding each saidcharge-collection electrode and also surrounding the field effecttransistor to which the respective charge-collection electrode iselectrically connected.
 12. A radiation detector as claimed in claim 11wherein each said field effect transistor has a gate electrode formed ona second surface of said layer of semiconductor material.
 13. Aradiation detector as claimed in claim 12 wherein the gate electrodes ofthe field effect transistors are connected to control lines for enablingsaid read-out means to output said output signal row-by-row orcolumn-by-column.
 14. A radiation detector as claimed in claim 1 whereinsaid read-out means comprises a plurality of semiconductor switchingdevices each for connecting a respective said charge-collectionelectrode of said plurality of first capacitors to a data line.
 15. Aradiation detector as claimed in claim 14 wherein said semiconductorswitching devices are field effect transistors, each said field effecttransistor being situated immediately below the respective saidcharge-collection electrode.
 16. A radiation detector as claimed inclaim 14 wherein said semiconductor switching devices are field effecttransistors, each said field effect transistor being situated adjacentto the respective said charge-collection electrode.
 17. A radiationdetector as claimed in claim 14 including means for electricallyisolating said first capacitors and said second capacitor from said dataline during a preset integration period during which electrical chargecan accumulate on said charge-collection electrodes.
 18. A radiationdetector as claimed in claim 1 including a layer of a first radiationconversion material for converting radiation to which that layer isexposed to optical radiation, and wherein said radiation conversionmeans of said second capacitor includes a further layer of a secondradiation conversion material for converting optical radiation producedin said layer of said first radiation conversion material to electricalcharge.
 19. A radiation detector as claimed in claim 18 wherein saidfirst radiation conversion material is a crystalline material having acolumnar structure.
 20. A radiation detector as claimed in claim 19wherein said first radiation conversion material is caesium iodide(CsI).
 21. A radiation detector as claimed in claim 1 wherein saidcharge-focussing means creates a plurality of potential wells in saidradiation conversion means, each said potential well being arranged tofocus charge onto a respective said charge-collection electrode.
 22. Aradiation detector as claimed in claim 1 comprising a plurality of diodepairs, the diodes in each pair being connected together back-to-back toa respective said charge-collection electrode.
 23. A radiation detectoras claimed in claim 1 wherein said charge-focussing means includes abody of semiconductor material so arranged as to focus said electricalcharge onto said charge-collection electrodes.
 24. A radiation detectoras claimed in claim 23 wherein said electrical charge focussed by saidbody of semiconductor material consists of electrons and said body ofsemiconductor material includes a body of p-type semiconductor materialproviding a charge-collection contact on each said charge-collectionelectrode.
 25. A radiation detector as claimed in claim 1 includingmeans for generating an electric-field across said second capacitor. 26.A radiation detector as claimed in claim 1 wherein the radiationconversion means is selected from the group consisting of:(a) group IVsemiconductors including silicon and germanium in crystalline,polycrystalline or amorphous form, (b) compound semiconductors includingGaAs, CdTe, HgI₂,TlBr in crystalline, polycrystalline or amorphous form,(c) semiconducting polymers, (d) amorphous selenium, (e) ionisationmedia including gas ion chambers containing gases such as Ar, Xe, Kr,gases with quench agents, and liquid ion Chambers, (f) gas proportionalchambers containing gases such as Ar, Xe, Kr, gases with suitable quenchagents, (g) liquid ion chambers, (h) scintillators in combination with aphotocathodes, (i) scintillators irradiating semiconductor layers, (j)metal foils having a substantial neutron interaction cross-section incombination with a device selected from (a) to (i) above, (k)piezo-electric material, and (l) semiconductor material operating ashall effect devices.
 27. An imaging apparatus incorporating a radiationdetector as claimed in claim
 1. 28. A medical X-radiation imagingapparatus incorporating a radiation detector as claimed in claim 1.